1. Field of the Invention
The present invention is directed to a method for shimming a magnet system in a magnetic resonance (MR) tomography apparatus, as well as to an MR tomography apparatus operating according to such a method.
2. Description of the Prior Art
In an MR tomography apparatus, the uniformity of the basic magnetic field is a critical factor. A specified magnetic field homogeneity is achieved in a defined volume on the basis of an appropriate magnet design and stationary shim measures (for example, with ferromagnetic materials applied at suitable locations). This volume, which is referred to below as the measuring volume, is typically a sphere having a diameter between 50 and 60 cm. In addition, shim coils, which further improve shimming the magnetic field, can be activated immediately before the measurement of the image data, and are normally present in an MR tomography apparatus. Usually, the entire nuclear magnetic resonance signal is employed for measuring the existing inhomogeneity, this being received by the antenna of the magnetic resonance apparatus after a non-localized excitation. Such shimming is also referred to below as xe2x80x9cglobalxe2x80x9d shimming.
U.S. Pat. No. 4,700,136 discloses that the field uniformity can be locally improved by using only the signal from the region wherein the actual measurement will later occur for setting the homogeneity. This region is referred to below as the target volume. A method that is suitable for local shimming of the basic magnetic field is disclosed, for example in U.S. Pat. No. 5,614,827. The previous goal of shimming was to compensate, by means of magnetic fields generated by the shim coils, inhomogeneities in the basic magnetic field during the entire data measurement. This local shimming, however, acts only on specific regions, i.e. a specific shim current setting can compensate only the inhomogeneity of the basic magnetic field in a specific region but cannot compensate it in others.
An object of the present invention is a number of simultaneously existing k-space paths, to reduce the influence of inhomogeneities of the basic field of the measured nuclear magnetic resonance signals.
The above object is achieved is achieved in accordance with the principles of the present invention in a method for shimming a magnet system of an MR tomography apparatus, and an MR tomography apparatus having at least one shim channel charged with a shim current, wherein each shim current is modified during the time of use of a k-space path for entering the raw data, obtained by scanning an examination subject, into k-space. Stated differently, in the method and MR apparatus of the invention, the shim currents (or at least one shim current) is modified between a time of excitation of nuclear spins in the examination subject, and a time of reading out the resulting magnetic resonance signals from the examination subject.
Due to the possibility of changing the shim current during the time of use of a particular k-space path, the possibility is provided of individually influencing various, simultaneously existing k-space paths, insofar as excitation times or readout times or the k-space paths differ. As used herein xe2x80x9ck-spacexe2x80x9d path means the time curve (sequence) of the signal positions (entries) in k-space, this being defined as follows:
kx=xcex3ƒGxxc2x7xxc2x7dt ky=xcex3ƒGyxc2x7yxc2x7dt; kz=xcex3ƒGzxc2x7zxc2x7dt
wherein Gx, Gy, Gz represent not only for the (desired) gradients generated by the gradient system but also include field inhomogeneities, and x, y, z are the coordinates of a Cartesian coordinate system, and xcex3 is the gyromagnetic constant. The invention is based on the perception that the basic field homogeneity is important only during the excitation and readout phases of an MR sequence. In the time intervals without excitation and readout, the time span wherein the spin dephasing is influenced is irrelevant. In general, it must be insured that the dephasing caused by the basic field inhomogeneity at the readout time is compensated by the shim fields as a time integral over the time interval between excitation and readout.